Electrodynamic pickup



Sept. 3, 1940. L. THOMPSON 2,213,936

I ELECTRODYNAMIC PICKUP Filed Jan. 13, 1939 INVENTOR Lincolnfhompsoni Patented Sept. 3, 194a UNITED STATES PATENT OFFICE The Sound Scriber Corporation, Stamford,

Conn., a corporation of Connecticut Application January s, 1939, Serial No. 250,729

5 Claims.-

This invention relates to phonographs, and

more particularly to an electrodynamic reproducer in which the needle holder carries a moving coil for actuating a loudspeaker.

One advantage of this construction is that no spring or other restoring member is required to maintain the needle holder between the magnetic poles, as is the case with electromagnetic pickups. Consequently, it is possible to provide a non-magnetic pickup of light metal, ,such as duralumin, being mounted so as to have only a minimum of stiffness, and requiring a relatively small amount of work to oscillate the needleholding structure. The weight applied to the needle to maintain it in the record groove without jumping may thus be diminished and this in turn greatly reduces the wear upon the sound grooves in the record. By means of the present improved pickup, it is possible to reproduce the recorded sounds with the utmost fidelity, using a needle pressure as low as to of an ounce, as compared with the customary needle pressures which are of the order of 2 ounces or more.

One object of this invention is to provide an improved pickup for use on disc records and the like, which will be inexpensive to manufacture, simple in construction, compact, ornamental in appearance, low in inertia, and very efficient and durable in use.

With these and other objects in view, which will appear as the description proceeds, there has been illustrated in the accompanying drawing, one form in which the invention may be con veniently embodied in practice.

In the drawing:

Fig. 1 is a schematic view showing the improved dynamic pickup as it appears during use.

Fig. 2 is a front view of the dynamic pickup per se.

Fig. 3 is a rear view of the same.

Fig. 4 is a bottom end view of the same.

Fig. 5 is an enlarged sectional view of the same, the section being taken on the line 55 of Fig. 2.

Fig. 6 is an enlarged fragmentary sectional view of the pickup, taken on the line B6 of Fig. 4.

Fig. 7 is a sectional view of on the line 1-1 of Fig. 5.

Fig. 8 is an enlarged perspective view of the vibratory needle and coil holding member-the coil being removed to show the interior construction.

Referring now to the drawing wherein like the pickup, taken reference numerals designate like parts throughout the several views, the numeral l0 denotes a casing in which the reproducer or pickup unit of this invention is mounted. The casing I0 is pivotally secured to and is carried by a reproducer arm I! pivoted on a vertical stud shaft l2, and said arm H is adapted to swing over a disc record l3 seated on a rotatable turntable- M which, is driven in any conventional manner, as by an electrical motor 10 it. The reproducer coil 52, to be described later, is connected by a pair of conductors I 6 and it to the input terminals l8 and I9 of a vacuum-tube amplifier of conventional form, and which is connected through a switch plug 2| to a pair of power line wires 22. The output terminals 23 and 24 of the amplifier 20 are connected by a pair of conductors 25 and 26 to a loudspeaker 21 of conventional form. The motor I5 is connected to the power lines 22 by a pair of conductors 28 and 29the circuit being controlled by a manually operated switch 3t.

The reproducer or pickup unit of this in vention includes a central permanently magnetized iron block 3|, and a pair of opposed L- shaped soft iron pole pieces 32 and 33 clamped against said block 3| by a pair of non-magnetic bolts 34 and 35. The bottom arms 36 and 31 of the pole pieces 32 and 33 are directed toward each other, and their adjacent ends 38 and 39 are spaced apart to form an air gap 40 in the magnetic circuit.

In order to centralize the pickup needle 48, provision is made of a small fiat vertical soft iron core member 4| fixed to a horizontal brass plate 42, which in turn is seated on the arms 36 and 31 of said polepieces 32 and 33 and covers the air gap 40.

The vibratory pickup needle 48 is detachably secured rigidly to a narrow rectangular frame member, preferably of light non-magnetic material, such as duralumin. The frame member comprises a pair of spaced-apart side members 43 and 44 and an integral offset end block 45. The block 45 has a hollow vertical stem 46 passing therethrough and preferably lying at right-angles to the plane of said frame. Provision is also made of a hollow interiorly threaded horizontal side stem. 41 extending at right-angles to the stem 46. The horizontal side stem 41 is adapted to receive a headed needle-clamping screw 49. The rear end of the frame member is connected to an offset black 50 which has a short, thin, horizontal tongue 5| extending rearwardly therefrom in substantial alinement with the axis of the needle clamping screw 49.

A small hollow coil of insulated wire 52 is seated upon and rigidly connected to the side members '43 and 44 of the frame member between the offset blocks 45 and 50 thereof. The interior bore-53 of the coil 52 is made slightly larger than the thin core member 4i, so that the needle 48 and its carrying frame will be permitted to move freely back and forth without contacting said core member when the needle 48 is oscillated by the lateral undulations of the sound groove in the disc record l3.

In order to provide a yieldable resilient pivotal mounting for the tongue SI of the needle-holding frame member, provision is made of a thin vertical strip of flexible soft rubber 54, or equivalent material, clamped to the rear sides of the pole piece arms 32 and 33 by means of a pair of clamp plates 55 and a pair of screws 58. The rubber strip 54 is provided with a slit I! to detachably receive the tongue 5i, which extends rearwardly from the needle-holding frame mem ber. To provide a yieldable bearing for the front of the needle holding frame member, the horizontal threaded stem 41 is surrounded by a soft rubber sleeve 58 secured in a brass plate 59 which is fastened by screws 60 to the side of the unit opposite that upon which the rubber strip 54 is secured. By means of this construction, the vibratory needle holding frame may be said to have a floating bearing in the air gap 40 of the magnetic circuit.

If desired, a counter balance weight 6i may be provided for the pickup head, said counter-balance extending rearwardly beyond the pivot point 62 on the reproducer arm ii. This permits the pressure of the needle 48 upon the disc record i3 to be maintained at a very low amount; 1. e., in the order of to ounces (5 to 15 grams). After the parts of the dynamic pickup have been assembled in the casing l0, Vaseline or the like will be preferably packed about the mov-- ing parts thereof to suppress or dampen any resonant vibrations that might otherwise occur.

Operation 39 of the pole pieces 32 and 33. The resulting induced electrical impulses set up in the coil 52 will be transmitted therefrom to the input terminals l8 and i3 of the amplifier 20 through the conductors i6 and ii. The amplified energy will be then transmitted to the loudspeaker 21, which will reproduce the original sound with high fidelity and with the minimum of distortion.

A further advantage of the present invention is that the reproducer needle, due to its light weight mounting, may be back-stepped transversely across the sound grooves without lifting the reproducer arm from the record and without in any way injuring or mutilating said grooves.

While there has been disclosed in this speciflcation one form in which the invention may be embodied, it is to be understood that this form is shown for the purpose of illustration only, and that the invention is not to be limited to the specific disclosure, but may be embodied and modified in various other forms without departing from its spirit. In short, the invention includes all the modifications and embodiments coming within the scope of the following claims.

Having thus fully described the invention, what is claimed as new. and for which it is desired to secure Letters Patent, is:

1. In an electrodynamic pickup fora grooved sound record, a U-shaped magnet having spaced opposed pole pieces, a loop shaped frame mounted for free oscillation. between saidpoie pieces, said 5 frame carrying a small electrodynamic conductor coil of. similar loop shape, a soft iron core member located within said frame and coil and out of contact therewith, and means to yieldingly cushion said coil holding frame to permit free vibration thereof.

2. In an electrodynamic pickup for a grooved sound record, a U-shaped magnet having spaced opposed pole pieces, a non-magnetic loop shaped frame mounted for free oscillation between said pole pieces, said frame carrying a small electrodynamic conductor coil of similar loop shape, a soft iron core member located within said'frame and coil and out of contact therewith, and means to yieldingly cushion said coil holding frame to permit free vibration thereof.

3. In an electrodynamic phonograph pickup for engagement in a grooved sound record, a permanent magnet having a pair of opposed spaced pole pieces, a fixed soft iron central core located between said pole pieces, a light weight vibratory loop-shaped frame pivotally and yieldably mounted between said pole pieces and freely surrounding said core, a loop-shaped loud-speaker-actuating coil of conductor wire secured to said frame for vibration therewith, said frame carrying a pickup needle extending downwardly substantially at right-angles to the plane thereof.

4. man electrodynamic pickup for a .grooved sound record, a loop magnet having spaced pole pieces, a light weight horizontal loop-shaped frame carrying a vertical needle at the bottom thereof for engaging the grooves of said record, a loop-shaped loud-speaker-actuating coil of conductive wire carried by said frame,said frame having at one end a hollow threaded stem for receiving a needle clamping set screw and at the opposite end a thin resilient horizontal tongue, and a soft rubber bearing for said stem and tongue respectively. v

5. In an electrodynamic phonograph pickup for engagement in a grooved sound record, a permanent magnet having a pair of opposed spaced pole pieces, a fixed soft iron central core located between said pole pieces and spaced therefrom by a surrounding air gap, a light weight nonmagnetic vibratory member pivotally and yieldably mounted in said air gap between said pole pieces and located in close proximity. to said core without contact therewith, an electrodynamic coil of conductor wire surrounding said core and secured to said member for vibration therewith, said member carrying a pickup needle extending downwardly therefrom.

LINCOLN THOMPSON. 

